netdev - Re: [PATCH bpf-next 1/3] libbpf: xsk: use bpf

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Date:   Mon, 22 Feb 2021 17:15:03 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        ciara.loftus@...el.com
Subject: Re: [PATCH bpf-next 1/3] libbpf: xsk: use bpf_link

On Tue, Feb 16, 2021 at 2:37 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> John Fastabend <john.fastabend@...il.com> writes:
>
> > Toke Høiland-Jørgensen wrote:
> >> John Fastabend <john.fastabend@...il.com> writes:
> >>
> >> >> > However, in libxdp we can solve the original problem in a different way,
> >> >> > and in fact I already suggested to Magnus that we should do this (see
> >> >> > [1]); so one way forward could be to address it during the merge in
> >> >> > libxdp? It should be possible to address the original issue (two
> >> >> > instances of xdpsock breaking each other when they exit), but
> >> >> > applications will still need to do an explicit unload operation before
> >> >> > exiting (i.e., the automatic detach on bpf_link fd closure will take
> >> >> > more work, and likely require extending the bpf_link kernel support)...
> >> >> >
> >> >>
> >> >> I'd say it's depending on the libbpf 1.0/libxdp merge timeframe. If
> >> >> we're months ahead, then I'd really like to see this in libbpf until the
> >> >> merge. However, I'll leave that for Magnus/you to decide!
> >> >
> >> > Did I miss some thread? What does this mean libbpf 1.0/libxdp merge?
> >>
> >> The idea is to keep libbpf focused on bpf, and move the AF_XDP stuff to
> >> libxdp (so the socket stuff in xsk.h). We're adding the existing code
> >> wholesale, and keeping API compatibility during the move, so all that's
> >> needed is adding -lxdp when compiling. And obviously the existing libbpf
> >> code isn't going anywhere until such a time as there's a general
> >> backwards compatibility-breaking deprecation in libbpf (which I believe
> >> Andrii is planning to do in an upcoming and as-of-yet unannounced v1.0
> >> release).
> >
> > OK, I would like to keep the basic XDP pieces in libbpf though. For example
> > bpf_program__attach_xdp(). This way we don't have one lib to attach
> > everything, but XDP.
>
> The details are still TDB; for now, we're just merging in the XSK code
> to the libxdp repo. I expect Andrii to announce his plans for the rest
> soonish. I wouldn't expect basic things like that to go away, though :)

Yeah, I'll probably post more details this week. Just catching up on
stuff after vacation.

As mentioned already, all the basic APIs (i.e., APIs like
bpf_program__attach_xdp and bpf_set_link_xdp_fd, though I hope we can
give the latter a better name) will stay intact. Stay tuned!

>
> >>
> >> While integrating the XSK code into libxdp we're trying to make it
> >> compatible with the rest of the library (i.e., multi-prog). Hence my
> >> preference to avoid introducing something that makes this harder :)
> >>
> >> -Toke
> >>
> >
> > OK that makes sense to me thanks. But, I'm missing something (maybe its
> > obvious to everyone else?).
> >
> > When you load an XDP program you should get a reference to it. And then
> > XDP program should never be unloaded until that id is removed right? It
> > may or may not have an xsk map. Why does adding/removing programs from
> > an associated map have any impact on the XDP program? That seems like
> > the buggy part to me. No other map behaves this way as far as I can
> > tell. Now if the program with the XDP reference closes without pinning
> > the map or otherwise doing something with it, sure the map gets destroyed
> > and any xsk sockets are lost.
>
> The original bug comes from the XSK code abstracting away the fact that
> an AF_XDP socket needs an XDP program on the interface to work; so if
> none exists, the library will just load a program that redirects into
> the socket. Which breaks since the xdpsock example application is trying
> to be nice and clean up after itself, by removing the XDP program when
> it's done with the socket, thus breaking any other programs using that
> XDP program. So this patch introduces proper synchronisation on both add
> and remove of the XDP program...
>
> -Toke
>